An ion source is known as an apparatus generating an ion beam by irradiation with a laser beam. An ion source increases energy of ions and outputs as an ion beam to an external device. An example of the external device is a heavy particle beam irradiation apparatus used for cancer treatment. A heavy particle beam irradiation apparatus is an apparatus accelerating an ion beam and irradiating a target site with the accelerated ion beam as a heavy particle beam. For example, carbon ions are used as the ions and, particularly, C6+ ions are used for cancer treatment.
For example, an ion source may employ a method using microwave discharge plasma and a method using a laser beam (for example, see Japanese Patent No. 3713524 and Japanese Patent Application Laid-Open No. 2009-37764, the entire content of which is incorporated herein by reference).
An ion source using a laser beam irradiates the surface of a target in a vacuum vessel with a laser beam and vaporizes and ionizes elements of the target with the energy of the laser beam to generate plasma (laser-ablation plasma). Ions included in the laser-ablation plasma are extracted from the vacuum vessel, and the extracted ions are accelerated at the time of extraction to generate an ion beam. The ion source using a laser beam can generate multi-charged ions such as C6+ ions by adjusting the energy and density of the laser beam.
In the ion source using a laser beam, unintended ions other than intended ions (C6+ ions) may be mixed in a vacuum vessel for the following reasons. For example, when moisture or particles are attached to a target, ions of contaminants (such as hydrogen molecular ion (H2+) and O8+ ions) may be mixed as unintended ions in the vacuum vessel. When residual gas is present in the vacuum vessel, ions of residual gas components may be mixed as unintended ions in the vacuum vessel. When unintended ions are mixed in the vacuum vessel, the unintended ions in addition to intended ions are extracted from the vacuum vessel and are output as an ion beam.